1. Field of the Invention
This invention relates in general to transmission lines, filter ring circuits and coupling circuits and in particular to a novel filter circuit for electrical waves.
2. Description of the Prior Art
In the transmission of information by means of electric communications transmission systems, the problem frequently occurs that only the signals lying within specific frequency bands are to be transmitted, with the simultaneous suppression of signals which lie in adjacent or adjoining frequency bands. For this purpose filters are used which, in the conventional technique, generally consist of coils and capacitors. These electric components possess the disadvantage, however that they possess relatively large dimensions in particular in the region of the lower frequencies, such as in the low frequency band and in the frequency band extending to a few hundred megahertz.
Therefore in particular in the field of the so-called integrated construction mode, new filter concepts have been investigated and in fact discovered. Until now these have basically consisted in the complete avoidance of coils where possible and by using appropriate transducers to derive the electric properties of coils from the electric behaviour of capacitors (gyrator filter). One has also adopted the procedure of forming appropriate filters from resistors and capacitors, possibly with interposed amplifiers in order to compensate the relatively high fundamental attenuation of such filters. In another group of modern filter circuits--the so-called computer filters--special logic linking circuits are used to influence the signals in frequency-dependent fashion. However, a common feature of all these and a series of other integrated filter concepts which have become known in the meantime is that they require a relatively high outlay.
In the context of very short electromagnetic waves, for examples having frequencies of above 2 to 3 GHz, a filter concept is known in which the electric signals are conducted to a line which is connected via a directional coupler to at least one enclosed line loop, a so-called line ring. An essential feature of this type of filter is that the coupling takes place via a directional coupler which undertakes input-coupling into the line loop in the one direction of circulation in as unweakened as possible a form or in accordance with a given coupling factor, whereas in the other direction of circulation of the line loop it substantially suppresses input coupling, or coupling. This type of filter has previously failed as far as lower frequencies are concerned, in that both an electric and a magnetic coupling are required to simulate the directional coupling, and that the line loop would generally have to possess much too large dimensions in order, for example to ensure the desired pass characteristics of the electric filter circuit in the low frequency band or in the band extending to a few Megahertz.